As soon as you get to the end of the trials and the urn SAVE!
Then select the urn and put the blood in the ashes. SACRILEGIOUS will pop. If you have Leliana with you she will immediately turn hostile and attack with the guardian. Simply die and reload your saved game (it will obviously be your latest) and do not put the blood in the urn this time. When you leave the room to the mountain top again the Dragon worshipers will attack you. They are not that difficult to kill and then CEREMONIALIST will pop too.
Both achievements quickly and easily. Just dont hit the gong or the alpha dog will fly down and tear you a new one...
* Read the comments below for variations and other information to these achievements. The information about coming up to this point will clarify if anyone was unsure.